Optical recirculation depolarizer and method of depolarizing light

ABSTRACT

An optical depolarizer and method of depolarizing light is described. An input light beam is split into two beams. One split beam is recirculated through a birefringent medium and looped back to be recombined with the input light. This allows a weighted averaging of the different polarization states that result from birefringence in the recirculation path of the recirculated beam. The depolarizer is formed from single mode fiber optic cables and fused single mode fiber couplers. Each fiber coupler has an input pair of fibers and an output pair of fibers. One of the output fibers is coupled to one of the input fibers to form a recirculation loop. Additionally, polarization controllers provided in the input fiber and recirculation loop allow the degree of polarization of the output beam to be varied across a wide spectrum of values.

BACKGROUND

1. Field of the Invention

The present invention relates to the field of optics in general. Moreparticularly, the present invention relates to the field of depolarizerswhich have applications in communications, sensors, optical instrumentsand other areas.

2. Description of the Related Art

Many optical devices used in communications and instrumentation, such asswitches, couplers and modulators, are highly sensitive to the state ofpolarization of light. The performance of communication systems andinstruments which utilize such optical devices varies as the state ofpolarization (hereinafter SOP) varies. Fluctuations in the SOP canresult in reduced signal to noise ratios in fiber optic communicationsystems or decreased sensitivity and accuracy in fiber opticinstruments.

When light passes through a fiber optic cable (hereinafter fiber), theinitial polarization of the light, whether polarized elliptically orlinearly, can be changed due to varying environmental factors affectingthe fiber. These environmental factors produce changes in the index ofrefraction of the fiber. Light propagating along the fiber will passthrough these regions having differing indexes of refraction, therebychanging the initial SOP of the light as it propagates along the fiber.This effect of altering the SOP of light as it passes through a mediumis called birefringence. The polarization received at the output end ofthe fiber may thus change radically from the initial SOP at the inputend of the fiber. Because birefringence is affected by varyingenvironmental factors, the output SOP will not have a predictablerelation to the input SOP. Instruments and devices sensitive to SOP willtherefore have their performance degraded in a manner which cannoteasily be predicted or corrected.

One solution to the problem of birefringence is to replace common singlemode fiber with polarization maintaining fiber (hereinafter PMF), whichis not sensitive to environmental factors and therefore preserves theinitial SOP as light propagates along the fiber. While PMF hasadvantages over standard single mode fiber, it is also very expensive touse. One meter of PMF costs approximately $10.00, roughly 100 times thecost of single mode fiber.

While single mode fiber has the drawback of birefringence, if theincident light propagating within the fiber is depolarized, then thebirefringent effect will not alter the SOP. Depolarized light is thecombination of light of all polarization states in equal proportion.Birefringence in single mode fiber alters all of the polarization statesequally, thereby preserving the depolarization of light propagatingalong the fiber. Thus, if depolarized light is used, birefringence nolonger produces a degradation in system performance.

One problem associated with utilizing depolarized light is that lightsources used in fiber optic systems have a high degree of polarization(hereinafter DOP). The DOP is defined as the fraction of optical powerthat is polarized. To utilize polarized light sources, a depolarizermust be employed to remove the DOP. Currently available depolarizershave significant limitations which reduce their practical applicabilityin both fiber optic communication systems and fiber opticinstrumentation.

One type of depolarizer is the electro-optic pseudo-depolarizer, whichutilizes electrodes positioned on either side of a waveguide to changethe refractive index within the waveguide. The varying refractive indexin turn varies the SOP of the light passing through the waveguide.Although varying the refractive index of the waveguide varies the SOP,the measured effective DOP depends on detector speed. Over severalcycles of varying the refractive index of the waveguide, thetime-averaged output light appears depolarized in that no one SOP ispreferred during the averaging time. This form of depolarization iscalled pseudo-depolarization or time-averaged depolarization, and hasthe disadvantage that light exiting the depolarizer within a narrow timeinterval has a high DOP. High speed detectors, however, detect light ina narrow time interval. Thus, a high speed detector would capture lightwith a high DOP when the light is time averaged over the narrow timeinterval. Additionally, the electro-optic pseudo-depolarizer is anactive system requiring both driving circuitry and a power supply.Failure of any of these active components would result in the lightexiting the waveguide with a high DOP. Another drawback of theelectro-optic pseudo-depolarizer is its high cost. An electro-opticpseudo-depolarizer costs approximately $1000.00.

Another type of currently available depolarizer is the acousticdepolarizer. A driving speaker vibrates a segment of fiber within thedepolarizer, thereby altering the index of refraction within the fiberas the fiber bends and vibrates. The index of refraction within thefiber varies at the frequency of the speaker. Polarized light passingthrough the vibrating fiber has its SOP altered at the frequency of thespeaker. As with the electro-optic pseudo-depolarizer, the acousticdepolarizer depolarizes light on a time averaged basis. The DOP thenvaries at the frequency of the driving speaker. For detectors andinstruments which detect polarization states at a time interval narrowerthan the time interval for depolarization, which is dependent on thefrequency of the speaker, light exiting the acoustic depolarizer willhave a noticeable DOP. Another disadvantage of the driving speakerdepolarizer is that it is an active system which relies on theperformance of the driving speaker. In addition to the costs associatedwith the driving speaker and associated circuitry, such a system isprone to failure if any of the many components of the speaker or drivingcircuitry fail. Thus, although the driving speaker depolarizer reducesthe DOP, the output light still retains a significant DOP within anarrow time interval.

Another known depolarizer is the Lyot depolarizer. The Lyot depolarizerconsists of two plates of quartz crystal having large retardances. Thelight source utilized with a Lyot depolarizer is a broad band source,for example a superluminescent diode. The crystals are arranged suchthat the incident light passes through the first crystal and into asecond crystal adjacent to the first crystal. The ratio of thickness ofthe two crystals is 2:1. While the light exiting the second crystal isdepolarized over a large wavelength region, light in a small wavelengthregion is not depolarized. Thus, the Lyot depolarizer is ineffective fordepolarizing monochromatic or narrow wavelength light sources. Anotherdrawback of the Lyot depolarizer is the high cost associated with usinga broad band light source. Broad band light sources have the additionaldisadvantage of having lower output power than is possible with narrowband light sources. Another disadvantage of the Lyot depolarizer is itsinapplicability with many fiber optic communication systems due to theuse of a broad band light source. As a pulse of light from a broad bandsource propagates along the birefringent single mode fiber used in manycommunications systems, there is a time shift in the pulse caused by thedifferent propagation speeds of the different wavelength components ofthe broad band light pulse. This time shift causes a "spreading" of thelight pulse and is incompatible with the high data transmission rates ofmany fiber optic communication systems.

Still another type of currently available depolarizer is described inU.S. Pat. No. 5,486,916, issued to Michal et al., and in U.S. Pat. No.5,457,756, issued to Hartl et al. This type of depolarizer isconstructed from PMF. The ends of the PMF are oriented such that theirprincipal axes are at an angle of 45°. In such a depolarizer the qualityof the depolarizer depends critically on the 45° alignment of the PMF.As with the Lyot depolarizer, this type of depolarizer also requires abroad band light source. Thus, in addition to the high cost of thedepolarizer dictated by the use of a broad band light source and the useof PMF, there are high fabrication costs associated with criticallyaligning and fusing the PMF. As with the time averaging pseudodepolarizers described above, this depolarizer spectrum averages and hasthe disadvantage in that it cannot be connected in series with otherdepolarizers of the same type. If it is connected in series with itself,as when two depolarizers are arranged such that the output of onedepolarizer is input a second depolarizer, the output light from thesecond depolarizer has its DOP increased from the output of the firstdepolarizer. Thus, the depolarizer described in U.S. Pat. No. 5,486,916is not suitable for producing very low DOP through series combination ofthe depolarizer.

Accordingly, it is desired that the present invention overcome thelimitations of current optical depolarizers.

SUMMARY OF THE INVENTION

The present invention provides an optical depolarizer and method ofdepolarizing light, wherein light is depolarized by splitting the beaminto an output beam and a recirculation beam. The recirculation beampropagates along a birefringent path and is then recombined with theinput beam before the input beam is split into the output beam andrecirculation beam. The combined input beam and recirculation beam isthen split into an output beam and a recirculation beam. This process ofsplitting, recirculating along a birefringent path and recombining withthe input beam averages beams with different states of polarization suchthat the output beam is the average of many light beams with differingstates of polarization.

In one embodiment of the present invention a birefringent element isused with a plurality of mirrors. The input light beam is split into twobeams by a partially reflecting mirror. One beam forms the output beamof the depolarizer. The other beam is reflected by mirrors through abirefringent element and back to the partially reflecting mirror. Thispart of the beam passing through the birefringent element forms therecirculation loop. Part of this beam striking the partially reflectingmirror is passed through as part of the output beam of the depolarizer.The other part is reflected along the path of the recirculation loop.

In one embodiment of the present invention the recirculation segment isused where the recirculating beam is reflected back along the path ofthe input beam and through a birefringent element along the input beam.The recirculating beam is then reflected back along the path of theinput beam in the direction of the input beam.

In one embodiment of the present invention a 2×2 fiber coupler is usedwhere one input fiber and one output fiber are connected to form arecirculation loop.

In one embodiment of the present invention polarization controllers areincluded within the input fiber and recirculation loop to allow thedegree of polarization to be tuned within a wide range of values.

In another embodiment of the present invention multiple single-ringdepolarizers of either the standard type or the tunable type areconnected is series such that the light output one single-ringdepolarizer is input the next single-ring depolarizer.

In another embodiment of the present invention multiple 2×2 fibercouplers are connected in a non-series arrangement to allowrecirculation and recombination of the light, thereby averagingdifferent polarization states and depolarizing the output light beam.

In another embodiment of the present invention a recirculationdepolarizer is formed from a single fiber coupled with itself to form arecirculation loop.

In another embodiment of the present invention a recirculationdepolarizer is formed as an integrated optical device on a substrate.Waveguides direct the input light beam along the recirculating path todepolarize the input light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a free space recirculating depolarizer with arecirculation loop in accordance with the present invention;

FIG. 2 is a diagram of a free space recirculating depolarizer with arecirculation segment in accordance with the present invention;

FIG. 3 is a diagram of a single-ring fiber optic recirculatingdepolarizer in accordance with the present invention;

FIG. 4 is diagram of three single-ring fiber optic recirculatingdepolarizers as shown in FIG. 3 connected in series in accordance withthe present invention;

FIG. 5 is a diagram of a single-ring tunable fiber optic recirculatingdepolarizer in accordance with the present invention;

FIG. 6 is a diagram of three tunable single-ring fiber opticrecirculating depolarizers as shown in FIG. 5 connected in series inaccordance with the present invention;

FIG. 7 is a diagram of three single-ring fiber optic recirculatingdepolarizers as shown in FIG. 3 connected in series with two tunablesingle-ring fiber optic recirculating depolarizers as shown in FIG. 5 inaccordance with the present invention;

FIG. 8 is a diagram of a multiple splitter recombination fiber opticdepolarizer in accordance with the present invention;

FIG. 9 shows a single ring depolarizer formed from a one piece of fiberlooped to form a recirculation loop in accordance with the presentinvention; and

FIG. 10 shows a recirculating depolarizer formed as an integratedoptical device in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method and apparatus for depolarizinglight. In the following description, numerous details are set forth inorder to enable a thorough understanding of the present invention.However, it will be understood by those of ordinary skill in the artthat these specific details are not required in order to practice theinvention. Well-known elements, devices, process steps and the like arenot set forth in detail in order to avoid obscuring the presentinvention. Further, all patents, technical papers and other referencesreferred to herein are incorporated by reference herein.

FIG. 1 is a diagram showing a free space depolarizer (2). Light from alight source, not shown, is input to a partially reflecting mirror (4).One part of the input beam is reflected from the partially reflectingmirror (4) and forms the output beam (6). The arrows along the path ofthe beam indicate the direction of propagation. The other part of theinput beam passes through the partially reflecting mirror (4) to amirror (8). The beam passing through the partially reflecting mirror (4)to the mirror (8) is referred to as the recirculation beam. The mirror(8) reflects the recirculation beam to a mirror (10). The recirculationbeam reflected off the mirror (10) then passes through a birefringentelement (12) and is reflected off a mirror (14). The birefringentelement (12) can be formed from any translucent material which altersthe SOP of a beam of light passing though it. After reflecting off themirror (14), the recirculation beam returns to the partially reflectingmirror (4). As before, the light beam intercepting the partiallyreflecting mirrors is split into two beams. One beam passes through thepartially reflecting mirror (4) and becomes part of the output beam (6)of the depolarizer (2). The other part of the recirculating beam isreflected by the partially reflecting mirror (4) to become part ofrecirculation beam.

In this manner, light input to the depolarizer (2) has a portion of thebeam passing through the birefringent element. The output beam of thedepolarizer (2) is then a combination of the input beam and therecirculation beams. The recirculation beam is continually split by thepartially reflecting mirror (4), such that part of the recirculationcontinually beam passes through the birefringent element. Each timelight passes through the birefringent element the SOP of the beamchanges. Thus, the output beam, as the combination of many beams ofdiffering SOP, no longer has a high DOP associated with the SOP of theinput light. From the addition of many recirculation beams withdifferent SOP, the resulting output beam has a low DOP.

The partially reflective mirror (4) can be chosen to allow any portionof the energy of the input beam to propagate along as the recirculationbeam. Possible successful embodiments include a 50/50 split where theinput beam is divided into two beams of equal intensity, as well as a67/33 split, where the first number represents the percentage of lightentering the recirculation beam and the second number represents thepercentage of light entering the output beam. A low DOP has been shownwith a 67/33 split, where 67% of the light reaching the beam splitter ispropagated along the recirculation loop.

While the embodiment shown in FIG. 1 utilized four mirrors and onebirefringent element, other embodiments could have several birefringentelements and a different arrangement of mirrors. Any arrangement ofmirrors and birefringent elements that allows the recirculation andrecombination of part of the input beam through a birefringent mediawould allow depolarization of the input light in accordance with thepresent invention.

FIG. 2 shows a diagram of a free space depolarizer (16) wherein therecirculation path is a linear segment. Light from a light source, notshown, is input to a two-way mirror (18), which allows the input beam topass through in the direction of propagation of the input beam. Afterpassing through the two-way mirror (18), the input beam passes through abirefringent element (20) which alters the SOP of the input beam. Afterpassing through the birefringent element (20), the input beam strikes apartially reflective mirror (22). The partially reflective mirror (22)splits the input beam into two beams. One part of the input beam passesthrough the partially reflective mirror (22) and becomes the output beamof the depolarizer (16). The other part of the input beam is reflectedback from the partially reflective mirror (22) as the recirculationbeam. The recirculation beam then backtraces the path of the input beam,first passing through the birefringent element (20), and then strikingthe two-way mirror (18). By backtracing it is meant that therecirculation beam reflected from the partially reflecting mirror (22)travels back in the direction of the two-way mirror (18). Therecirculation beam need not retrace the exact path of the beam. Afterstriking the two-way mirror (18), the recirculation beam is thenreflected back from the two-way mirror (18), through the birefringentelement (20), to strike the partially reflective mirror (22). As before,the recirculation beam is continually reflected between the mirrors. Inthe depolarizer (16) the path between the two-way mirror (18) and thepartially reflective mirror (22), including the birefringent element(20), is the recirculation path of the depolarizer. Because therecirculation path of this depolarizer follows the path of the inputbeam, rather than a separate recirculation loop as in the depolarizer(2) shown in FIG. 1, the recirculation path is referred to as arecirculation segment.

A polarized input light beam entering the depolarizer (16) will have itsDOP reduced in the following manner. As the input beam passes throughthe depolarizer (16) along the recirculation segment, the SOP of theinput beam will be altered due to the birefringence of the recirculationsegment, but the DOP will remain the same. The recirculation beams whichtraverse back along the recirculation segment will have their SOP variedfrom the SOP of the input beam due to the birefringence of therecirculation segment. After the recirculation beam is reflected backfrom the two-way mirror (18), the input beam propagating through thetwo-way mirror, and the recirculation beam reflected from the two-waymirror, combine to form a beam of decreased DOP, as the combination oftwo beams with different SOP. When the combined beam strikes thepartially reflecting mirror (22), the reflected beam returns along therecirculation segment and is reflected by the two-way mirror (18). Thecombined beam has a lower DOP than the input beam. The part of thecombined beam passing through the partially reflective mirror (22) has alower DOP with each successive addition of a recirculation beam.

Although the above embodiments show a depolarizer with a birefringentelement separate from the mirrors of the depolarizer, other embodimentscould have the birefringent element as part of the mirror or mirrors ofthe depolarizer.

While the embodiment shown in FIG. 2 uses separate mirrors and aseparate birefringent element, one way of constructing such adepolarizer includes using a single fiber which has been grated. Asection of the fiber is grated to produce a first grating which allowsthe input beam traveling along the fiber to pass through. Further alongthe fiber in the direction of propagation of the input beam a secondgrated section is formed. This second grated section reflects a portionof the beam striking it, while allowing a portion of the beam to passthrough. The segment of fiber between the first grating section and thesecond grating section acts as a birefringent element to alter the SOPof light which propagates along it. The first grated section, the secondgrated section and the segment of fiber between the grated sections formthe recirculation segment. The input beam of the fiber passes throughthe first grated section to the second grated section. The input beamreflected back from the second grated section along the fiber to thefirst grated section, where the light is reflected again. When the lightis reflected back from the first grated section it is combined with theinput beam traveling along the fiber. A portion of this combined beampasses through the second grated section and forms the output beam ofthe depolarizer. As before, a portion of the combined beam is reflectedback from the second grated section to the first grated section, therebyrepeating the process of splitting and recombining with the input beam.The combining of beams with different SOP reduces the DOP of the outputbeam.

FIG. 3 is a diagram showing a single-ring recirculating depolarizer (24)constructed from a standard 2×2 fiber coupler (28) which are availablefrom Gould and AMP. A 2×2 fiber coupler acts both as a standard Ycoupler (29) and as a beam splitter (31). Light beams entering the fibercoupler through its two input fibers are combined by a standard opticalcoupler to form a combined beam. The combined beam then propagates alongfiber within the fiber coupler to the beam splitter of the fibercoupler. The beam splitter divides the combined beam into two beams.These two beams exit the fiber coupler through its two output fibers.

In the diagram of the depolarizer (24) shown in FIG. 3, light from alight source, not shown, is directed into the input fiber (26) of the2×2 fiber coupler (28). The light source can be any coherent lightsource, such as a laser diode. One of the two beams split from the inputbeam exits the fiber coupler (28) through the output fiber (30). Thisexiting beam will hereafter be referred to as the output beam. The othersplit beam exits the fiber coupler (28) though the fiber (34) whichforms part of the recirculation loop (32). The recirculation loop (32)may be formed by coupling one of the output fibers of the fiber couplerto one of the input fibers.

Thus, light entering the depolarizer (24) is sent along the input fiber(26) to the fiber coupler (28) where the input beam is split by the beamsplitter into two beams. One of the split beams is sent along the outputfiber (30) and the other split beam is sent along the recirculation loop(32). The beam sent on the recirculation loop (32), which will bereferred to as the recirculation beam, is then recombined with the inputbeam within the fiber coupler (28). This combined beam of light formedfrom combining the input beam and the recirculation beam is then splitby the splitter within the fiber coupler into two beams, as before, oneof which is sent along the output fiber (30) and one of which is sentalong the recirculation loop (32). Thus, light from the input beam isdivided, recirculated, combined with the input beam, divided,recirculated, recombined with the input beam, and so on.

When considering the effect on the SOP of the output beam (30) due tothe recirculation and recombination of the input beam (26), thefollowing occurs. Light passing through a fiber experiencesbirefringence, i.e. the SOP changes as the light propagates along thefiber. Thus, the initial SOP is altered each time light passes through afiber. In the depolarizer shown in FIG. 3 the initial SOP of the inputbeam (26) is altered as the beam passes through the fiber coupler (28)and as the beam passes along both the recirculation loop and the fiberof the output beam (30). While the initial output beam (the output beambefore recirculation beams are combined with the input beam) will have adifferent SOP than the input beam, the degree of polarization (DOP) isthe same. Each of the input and output beams has equal DOP but adifferent orientation, or SOP. As the recirculation beam (32) isrecombined with the input beam (26), the SOP of the light of therecirculation beam is different than the SOP of the input beam due tothe birefringence inherent in the fiber coupler (28) and therecirculation loop (32). Even as the birefringent effect changes withchanging environmental factors, the SOP of the recirculation beam willcontinue to be altered from the SOP of the input beam.

Thus, the combined beam, formed from coupling the input beam and therecirculation beam within the fiber coupler, will no longer be polarizedin one particular direction, but will be the combination of two beamswith different polarization states. As explained above, this combinedbeam is split into two beams by the beam splitter within the fibercoupler (28). This new output beam, being the combination of the inputbeam and the recirculation beam, will then have not only a different SOPthan the input beam but, as the combination of beams of different SOP,will also have a different DOP since this new output beam will no longerbe polarized in one SOP. Similarly, the recirculation beam resultingfrom the splitting of the combined input beam and recirculation beamwill also not have the high DOP of the initial input beam.

This process is repeated continually as the beam within the depolarizeris continually split, sent along the recirculation loop, recombined withthe input beam, and input into the beam splitter. The net result of eachsplit, recirculation and recombination is that the output beam is thecombination of many beams of differing SOP and DOP, thereby reducing thedegree of polarization of the output beam with each recombination.

If the recirculation loop is longer than the coherence length of thelight of the input beam, then the input beam will not be coherent withthe recirculation beam. The coherence length of a light source is a wellknown quantity and is proportional to the inverse of the spectrallength. When two beams that are not coherent to each other are combined,they do not substantially interfere with each other. For a standardlaser diode with a wavelength of 1300 nm and a spectral length of 0.1nm, the coherence length is approximately 1.67 cm. Thus, as long as therecirculation loop is longer than 1.67 cm, the interferometric effectsof combining the input beam and the recirculating beam will benegligible. As n, the number of recirculations, approaches infinity,i.e. when the depolarizer has been connected to the light source formore than a few nanoseconds, the intensity of the light approaches theintensity of the input beam, assuming no internal or splicing losses.

FIG. 4 is a diagram illustrating three single-ring recirculatingdepolarizers of FIG. 3 connected in series such that the output from afirst depolarizer (36) is the input of a second depolarizer (38) and theoutput of the second depolarizer (38) is the input of a thirddepolarizer (40). The intensity of the output beam (42) is equal to theintensity of the input beam, assuming no internal losses or splicinglosses. Thus, even with three single-ring depolarizers connected inseries, the output beam has the same intensity as the input beam.

The embodiments shown in FIGS. 1 and 2, including the fiber gratingembodiment of the depolarizer of FIG. 2, may also be connected in seriesas shown in FIG. 4.

FIG. 5 shows a tunable single-ring depolarizer (44), where apolarization controller (46) is included in the input fiber (48). Thepolarization controller (46) is adjustable to control the polarizationof the input beam within the input fiber (48). Light from the inputfiber (48) that passes through the polarization controller (46) is inputto the 2×2 fiber coupler (50). The fiber coupler (50) splits the inputbeam from the input fiber (48) into two beams, which exit the fibercoupler (50) through fibers (52) and (54). Fiber (52) carries the outputbeam and fiber (54) connects to a polarization controller (56).Polarization controller (56) is also connected to the input fiber (58)of the fiber coupler (50). In this manner the polarization controller(56) and fibers (54) and (58) form the recirculation loop (60) of thetunable single-ring depolarizer (44). After passing through therecirculation loop (60), light exiting the fiber coupler (50) throughfiber (54) is recombined with the input beam from fiber (48) by thefiber coupler (50).

By adjusting both the polarization of the input beam by the polarizationcontroller (46) and the polarization of the recirculation beam by thepolarization controller (56), the DOP of the output beam can be tuned tosignificantly lower DOP than was realizable with the single-ringdepolarizer (24) shown in FIG. 3. The tunable single-ring depolarizer(44) of FIG. 5 has been found to depolarize light to as low as 1.15%.Significantly, the tunable single-ring depolarizer (44) may be adjustedto provide an output beam with a 99.8% polarization. Thus, the tunablesingle-ring depolarizer is able not only to provide a highly depolarizedlight, but can also be tuned such that it provides nearly perfectlypolarized light. This gives the tunable single-ring recirculationdepolarizer great flexibility in applications were the DOP of the lightwould need to be varied or specifically tuned to a particular DOP.

FIG. 6 illustrates three tunable single-ring depolarizers of FIG. 5connected in series. The output of one tunable single-ring depolarizeris connected to the input of the next tunable single-ring depolarizer.Each tunable single-ring depolarizer (64), (66) and (68) is constructedin the same manner as the tunable single-ring depolarizer (44) shown inFIG. 5, which includes polarization controllers (46) and (56) insertedin the input fiber (48) and recirculation loop (60) of the depolarizer(44). By utilizing three tunable singlering depolarizers connected inseries, the degree of polarization of the light output from the thirdtunable single-ring depolarizer has been reduced as low as -20 dB.

Several possible types of polarization controllers could be utilizedwith the depolarizer (44). One possible depolarizer is described in U.S.Pat. No. 4,389,090 issued to H. C. LeFevre, which is incorporated byreference herein. This type of polarization controller utilizes twometal plates to twist two coils of fiber, thereby inducing birefringenceto change, and thereby control, the polarization of light output fromthe polarization controller. Other possible polarization controllersinclude liquid crystal polarization controllers and integrated opticspolarization controllers.

While the embodiments of multiple single-ring depolarizers shown in FIG.4 and FIG. 6 are constructed from either the single-ring depolarizer(24) shown in FIG. 3 or the tunable single-ring depolarizer (44) shownin FIG. 5, other embodiments of the present invention could combinedepolarizers of the type (24) shown in FIG. 3 with tunable depolarizers(44) of the type shown in FIG. 5. Additionally, other embodiments of thepresent invention could combine more or fewer single-ring depolarizersof either type, or both types, in any order. As one possible example ofthis, the depolarizer (70) shown in FIG. 7 is constructed from threesingle-ring depolarizers (72), (74) and (76) of the type shown in FIG. 3and two tunable single-ring depolarizers (78) and (80) of the type shownin FIG. 5. In the embodiment shown in FIG. 7, the tunable single-ringdepolarizers (78) and (80) occupy the third and fifth positions in theorder of depolarizers along the path of propagation of the light beamthrough the fiber from left to right.

While the above multiple single-ring depolarizers connected single-ringdepolarizers in series, other embodiments of the present invention couldconnect recirculating depolarizers in different configurations.

Specifically, FIG. 8 is a diagram illustrating a multiple splitterrecombination depolarizer (82) constructed from four 2×2 fiber couplers(84), (86), (88) and (90). Light is input to the fiber coupler (84)through input fiber (92). The fiber coupler (84) splits the beam fromthe input fiber into two beams. The two beams exiting the fiber coupler(84) are connected to the fibers (94) and (96). Fiber (94) is inputfiber coupler (90) and fiber (96) is input fiber coupler (86). Fibercoupler (86) recombines beams of light from fibers (108) and (96) andthen splits the recombined beam into two beams. These two beams exitfiber coupler (86) through exit fibers (98) and (100). Fiber (98)connects to the input of fiber coupler (88). Fiber (100) serves as theoutput fiber of the depolarizer (82). The output beam (98) from fibercoupler (86) is combined by the fiber coupler (88) with the beamconnected from the fiber (102). Fiber coupler (88) splits the combinedbeam from fibers (98) and (102) into two beams. These two beams exitfiber coupler (88) through fibers (104) and (106). Fiber (104) is inputfiber coupler (84), fiber (106) is input fiber coupler (90). Fibercoupler (90) combines the beams of light from fibers (94) and (106) andthen splits the combined beam into two beams. These two beams exit fibercoupler (90) through fibers (102) and (108). Fiber (108) connects tofiber coupler (86) and fiber (102) connects to fiber coupler (88). Fibercoupler (84) combines the beam from fiber (104) with the beam from inputfiber (92). Fiber coupler (84) then splits the combined beam into twobeams which exit through fibers (94) and (96).

Polarized light entering the depolarizer (82) through input fiber (92)is repeatedly split and recombined with recirculated light beams as thesplit and recombined beams circulate through the four fiber couplers(84), (86), (88) and (90). In this manner the output beam (100), whichis a combination of recirculated beams, is then the weighted average ofthe polarization states of all of the combined beams. Due to thebirefringence of light traveling along the fibers between the fibercouplers, the result of the weighted averaging of the polarizationstates of the combined beams produces an output beam with a very lowDOP.

While the embodiment shown in FIG. 8 utilizes four fiber couplers, otherembodiments could utilize more or fewer fiber couplers arranged suchthat the light beam traveling through the depolarizer is repeatedlysplit, sent along different optical paths with different birefringenteffects, and recombined to average the different polarization states,thereby depolarizing the output light beam. Additionally, otherembodiments of the present invention may use polarization controllers orsingle-ring depolarizers with multiple fiber couplers connected in anon-series manner. The embodiment illustrated in FIG. 8 is provided todemonstrate the versatility of the present invention. As such, it is butone of many possible combinations in accordance with the presentinvention.

While the above embodiments shown in FIGS. 2-8 utilized a standardnarrow band laser diode, the present invention can also utilize broadband light sources depending on the application. This flexibility of thepresent invention, to use either broad band light sources or narrow bandlight sources, with their lower cost and greater reliability, gives thepresent invention an advantage over currently available opticaldepolarizers.

While the above embodiments shown in FIGS. 2-8 utilize standard 2×2fiber couplers due to their low cost and low internal loss, otherembodiments of the present invention may utilize separate beam splittersand couplers to split and recombine the light beam. Other embodimentsmay utilize 3×3 or 4×4 or greater fiber couplers to allow larger numbersof recirculation loops. While the fiber couplers of FIGS. 3-4incorporated a 50/50 beam splitter, this is used as an example only andother embodiments could utilize splitters with different splittingratios. While the recirculation loops of the embodiments shown in FIGS.3-4 are formed by splicing two fibers together, in other embodiments therecirculation loop could be formed from a single fiber.

FIG. 9 shows a single ring depolarizer (120) formed from a single pieceof fiber (110) looped to form a recirculating loop (112). Light inputthe fiber (110), forming the input beam, propagates along the fiber andthrough the recirculation loop (112) to the coupling point (114) wherethe fiber (110) is coupled with itself. Light propagating along therecirculation loop (112) is split into two beams at the coupling point(114). The beam propagating along the recirculation loop (112) isreferred to as the recirculation beam. One beam propagates along theoutput end (116) of the fiber to form the output beam of the depolarizer(120). The other beam propagates along the recirculation loop (112) ofthe fiber (110). The birefringence of the recirculating loop alters theSOP of recirculation beam from the SOP of the input beam. Combing therecirculation beam with the input beam at the coupling point (114)results in a lowering of the DOP of the output beam of the depolarizer(120).

The coupling of the fiber (110) with itself can be performed by mergingthe fibers together, typically after some of the fiber's cladding hasbeen removed, to allow light to pass between the light carrying cores.Processes including heating, polishing or chemical merging may beemployed to this end.

While the embodiments disclosed herein used single mode fiber, thepresent invention is not limited to single mode fiber and otherembodiments could be constructed from other types of optical fiber.Additionally, several different types of optical fiber can be used inone depolarizer.

Additionally, while the present invention utilizes single mode fibersdue to their low cost and availability, other embodiments of the presentinvention may utilize other types of fiber or other media to recirculateand transmit the beam path.

FIG. 10 shows a depolarizer (122) formed as part of an integratedoptical chip (124). Several materials are available for waveguideformation, such as LiNbO₃ or plastics. Light from an input source, notshown, is directed into the input waveguide (126) and forms the inputbeam. The input beam propagates along the waveguide (126) to the point(128). At the point (128) the waveguide splits into a output waveguide(130) and a recirculation waveguide (132). The input beam is split intotwo beams when it reaches the point (128). One of the split beamspropagates along the output waveguide (130) and forms the output beam ofthe depolarizer (122). The other split beam propagates along therecirculation waveguide (132) and forms the recirculation beam. Apolarization controller (136) is included in the recirculating waveguideand alters the SOP of the recirculation beam. The recirculation beam iscombined with the input beam at the point (134). The combinedrecirculating beam and input beam propagates to the point (128) where itis split into two beams, as described above. The output beam propagatingalong the output waveguide (130) is then formed from the combination ofthe input beam and the recirculation beam.

In this way a portion of the input beam is diverted along arecirculation path which, due to the polarization controller (136),alters the SOP of the light. When the recirculation beam is combinedwith the input beam, the DOP of the combined beam is lower than the DOPof the input beam. Each time the combined beam reaches the point (128),a portion of the combined beam is diverted along the recirculationwaveguide (132). Each time the beam from the recirculation waveguide(132) is combined with the input beam, the DOP of the resulting combinedbeam is lowered due to the change in the SOP of the beam propagatingalong the recirculating waveguide. The resulting output beam of thedepolarizer, as the combination of beams with different SOP, has a lowerDOP than the input beam.

The polarization controller (136) can be of either an electric field oracoustic type. In the electric field or acoustic type polarizationcontroller either an electromagnetic wave or an acoustic wave is used tochange the index of refraction of a medium within the polarizationcontroller, thereby altering the SOP of a beam of light passing thoughtthe polarization controller.

While the embodiment shown in FIG. 10 utilizes a waveguides formed on asubstrate, other embodiments could have waveguides which are formed in asubstrate by etching and deposition methods or other processes.Practitioners of the art of integrated optics will appreciate the manymaterials and methods used in forming integrated optical devices whichare applicable in forming an integrated optical depolarizer inaccordance with the present invention. As such, the embodiment shown inFIG. 10 is given as a representative example and is not meant to belimiting on the materials or methods used in forming an integratedoptical depolarizer.

While the embodiment shown in FIG. 10 has only one recirculation loop,other embodiments could have multiple recirculation loops and multiplepolarization controllers. In an embodiment with multiple recirculationloops, such loops may be positioned on both sides on the input andoutput waveguides, as one loop is formed on one side in FIG. 10. Inanother possible arrangement, multiple loops could be positioned on thesame side of the input and output waveguides. Additionally, thedepolarizer (122) of FIG. 10 can be connected in series with otherdepolarizers formed on the same substrate or on separate substrates.

While the above embodiments have assumed negligent splicing losses andinternal losses for explanation purposes, as with all optic systems anddevices, the actual losses of a depolarizer in accordance with thepresent invention will vary depending on the materials selected as wellas the methods and quality of construction.

Although the invention has been described in conjunction with particularembodiments, it will be appreciated that various modifications andalterations may be made by those skilled in the art without departingfrom the spirit and scope of the invention. In particular, those skilledin the art will recognize that the present invention is not limitedfiber optic communications and optical instruments.

What is claimed is:
 1. An optical depolarizer comprising:an input forreceiving an input light; a splitter; a recirculation loop having a loopoutput, wherein the input light is split by the splitter into at leasttwo beams and wherein one of the at least two beams is directed alongthe recirculation loop; and means for combining the loop output with theinput light.
 2. The depolarizer of claim 1, wherein a combined output ofthe means for combining is input to the splitter.
 3. The depolarizer ofclaim 1, wherein a first splitter output is input to a loop input of therecirculation loop.
 4. The depolarizer of claim 2, wherein a firstsplitter output is input to a loop input of the recirculation loop. 5.The depolarizer of claim 1, further comprising:a polarization controllercoupled to the input of the depolarizer.
 6. The depolarizer of claim 1,further comprising:a polarization controller inserted within therecirculation loop.
 7. An optical depolarizer, comprising:a N×N fibercoupler having N inputs and N outputs; and a recirculation loop formedfrom coupling at least one output with at least one input.
 8. Thedepolarizer of claim 7, further comprising:a first polarizationcontroller coupled to the input of the depolarizer; and a secondpolarization controller inserted within the recirculation loop.
 9. Anoptical depolarizer, comprising:a plurality of N×N fiber couplers, eachfiber coupler having N inputs and N outputs; and at least onerecirculation loop formed by coupling at least one output to at leastone input.
 10. The depolarizer of claim 9, wherein the at least onerecirculation loop couples at least one output to at least one input ofthe same fiber coupler.
 11. The depolarizer of claim 9, furthercomprising:at least one polarization controller coupled to an input ofthe at least one fiber coupler; and at least one polarization controllercoupled to the at least one recirculation loop.
 12. The depolarizer ofclaim 9, wherein N=2.
 13. A method for depolarizing a light source,comprising the steps of:splitting an input light beam into a pluralityof light beams; recirculating at least one of the plurality of lightbeams along a birefringent path; and combining at least one of therecirculated light beams with the input light beam.
 14. The method ofclaim 13, wherein the recirculated light beam is combined before theinput light source is split into a plurality of light beams.
 15. Themethod of claim 13, wherein the polarization of the input light beam iscontrolled by a polarization controller.
 16. The method of claim 13,wherein the polarization of the recirculated light beam is controlled bya polarization controller.